The filler pipe of a motor vehicle is typically provided in an inlet box formed by recessing a part of the outer panel of the vehicle body, and is typically fitted with a seal member that engages a filler nozzle when it is placed in the filler pipe for the purpose of preventing the release of fuel vapor to the atmosphere at the time of refueling (see U.S. Pat. No. 5,035,272 (Kawase) issued Jul. 30, 1991). Such a filler pipe is also provided with a relief valve for releasing excess pressure in the filler pipe. For instance, when the filler nozzle is improperly placed in the filler pipe and the level sensor incorporated in the filler nozzle fails to detect the rise of the fuel level, it becomes necessary to release the resulting excess pressure. In the invention disclosed in the aforementioned US patent, a seal lip that serves as such a relief valve is integrally formed with the seal member.
However, according to a conventional structure, the metallic filler nozzle engages the seal surface of the seal member made of elastomeric material and applies a significant load on the seal member. The seal member is required to have a certain rigidity for an improved mechanical strength and wear resistance, but must be pliant enough to achieve a required sealing performance, and it is not possible to meet the two requirements at the same time.
It is also known to provide a relief valve outside the filler pipe separately from the seal member. However, because of a limited space in the filler pipe, there is a significant difficulty in accommodating the necessary components and piping for such a relief valve in the available space. Also, there must be an adequate space or a protective member to prevent the relief valve from damages even at the time of a crash or a rollover. An increase in the number of component parts also adds to the difficulty in ensuring an adequate sealing for fuel vapor.